1. Field of the Invention
This invention relates to a smoke detector for detecting attenuation or interruption of intermittently emitted light pulses for detecting smoke generated by fire.
2. Prior Art
There is already known a separate-type smoke detector in which a light emitted section and a light receiving section are disposed opposite to each other and spaced at a given distance to define a detection area therebetween for detecting attenuation or interception of light pulses which is intermittently emitted from the light emitting section. The smoke detector is provided with a monitor terminal for producing a monitor signal corresponding to a photo-signal for the purpose of adjusting operation of the optic axis and the photo-output level when the detector is installed. For testing and adjusting, a measuring apparatus such as an ammeter is connected externally and adjustment is carried out by measuring the monitor signal.
In the smoke detector of this type, however, emission of light pulses is made, for example, at intervals of 3 seconds in order to save power consumption. Therefore, if the monitoring period is not changed for test and adjustment, there is caused a time delay corresponding to the monitoring period before a change appears in the output signal at the monitor terminal after the adjusting operation has been made. Thus, the conventional smoke detector of this type has the problem that it takes too much time to carry out the adjustment operation.
The conventional smoke detector also has another problem. When the light emitting section and the light receiving section of the smoke detector are set on the ceiling or the like, the adjustment of the optic axis between the light emitting section and the light receiving section is first made and thereafter the photo-output level adjustment is made to set the level of the photo-signal to a desired level under the condition when there is no smoke in the detection area. However, in the conventional smoke detector, the light receiving section and the light emitting section are supplied with power from a central signal station which is in operation while the adjustment operation is carried out. Consequently, if during adjustment the photo-detector of the light receiving section is inadvertently shut off, the level of the photo-signal falls below a threshold level and a false fire detection signal is transmitted to the central signal station. Since a plurality of smoke detectors are connected to the same line derived from the central signal station, and a fire signal is generated when the impedance of the signal line from the central signal station falls to a low value, the central station will be unable to receive a true fire detection signal if a smoke detector other than the detector under adjustment detects a fire. Or, the alarm system may be set off so as to prevent the central signal station from operating when a false fire detection signal is produced during the adjustment operation. In either case, normal supervisory operation cannot be maintained.